Satellite positioning has been widely used in various applications. In satellite positioning systems such as Global Navigation Satellite System (GNSS), a cold start state means that no information such as receiver position, observation time, satellite navigation information (e.g. almanac or ephemeris) are available.
For a GNSS (e.g. GPS) receiver, sensitivity is a major performance criterion. TTFF (Time to First Fix) under the cold start state is a representative standard for receiver sensitivity. To fix a three dimensional position, at least four satellites are required. The TTFF depends on time to acquire satellite signals of at least four visible satellites and time to download required ephemeris by demodulating the navigation data. When the received signal power is efficiently strong, it takes a few seconds to acquire the satellite signals and about 27 seconds to obtain the required ephemeris. If the signal power is weak, the time to acquire the satellite signals and the time to download the ephemeris are both extended. No matter the signal power is strong or weak, it is apparent that the time to obtain the ephemeris is a dominant factor of TTFF.
The satellite navigation data is transmitted in pages. Five subframes make a page (or a frame). Each subframe requires 6 seconds to transmit. Accordingly, to transmit a page, 30 seconds are required. Each subframe contains ten words, which will be further described in detail later. FIG. 1 schematically and generally shows data structure of one page of the navigation data. Satellite Navigation data include ephemeris and almanac. Each satellite broadcasts its own ephemeris. In addition, each satellite transmits almanac of all satellites, which is a coarse version of the ephemeris. The essential satellite ephemeris and clock parameters are transmitted in Subframe 1, Subframe 2 and Subframe 3. Almanac is transmitted in Subframe 4 and Subframe 5. As shown in FIG. 2, a complete almanac is distributed into Subframes 4 and 5 of 25 pages. To transmit 25 pages of the navigation data, 12.5 seconds are required. Information carried in Subframes 1-3 is the same for these 25 pages if no update occurs. In other words, the information in Subframes 1-3 is repeated once per 30 seconds. It is necessary to collect Subframes 1 to 3 so as to obtain a complete ephemeris of the satellite. In general, ephemeris of one satellite is updated per two hours.
FIG. 3 is a flow chart showing a conventional process for collecting ephemeris. The process starts in step S10. When a receiver receives a subframe SF(i) (step S20), the receiver checks the ten words of the subframe SF(i) to see if these ten words are correct by checking parity of each word (step S30). If parity check indicates an error in any of the ten words, this subframe SF(i) is discarded. If the ten words are all correct, then this subframe SF(i) is passed to the next step. In step S40, it is checked if this subframe SF(i) is one of Subframes 1 to 3. If so, this subframe SF(i) is stored in a buffer; in addition, flag SF(i)_collected is set (step S50). In step S60, it is checked if Subframes 1 to 3 are all collected. That is, it is checked if the complete ephemeris is obtained. If so, a navigation database is updated (step S70). If not, the whole process should be repeated. Briefly, when there is an error among the ten words of a received subframe, the subframe is discarded. If, unfortunately, the subframe of the next page also has an error, the receiver again needs to wait for another 30 seconds. This results in considerably long time consumption, and thereby seriously influencing TTFF performance of the receiver.